fix: correct the relation between the in-plane shear strain and the twisting curvature

[mortenengen]

With this fix, the relation between generalized strains and in-plane strains at a level z becomes:

$$ \begin{bmatrix}\epsilon_x\cr\epsilon_y\cr\epsilon_{xy}\cr\end{bmatrix} = \begin{bmatrix}\epsilon_{x0}\cr\epsilon_{y0}\cr\epsilon_{xy0}\cr\end{bmatrix} - z \begin{bmatrix}\chi_{x0}\cr\chi_{y0}\cr2\chi_{xy0}\cr\end{bmatrix} $$

where the subscript zero indicates the generalized strains at the mid-plane. Before this fix, the factor 2 in the expression was missing, and the relation between the twisting moment and the twisting curvature was therefore not correct.